In return-to-zero (RZ) coding, the frequency spectrum of a coded signal will include a strong peak at the clock frequency. Clock extraction can then be achieved by filtering at the clock frequency and rectifying the result. However, this involves signal conversion to electronic form. It is much preferable to be able to extract the clock frequency by optical means.
Demand for broadband services (such as high quality data transfer, high definition television and video conferencing) may require telecommunications networks to operate with TBit/s capacities by the first decade of the next century. In order to meet this capacity demand, all-optical or "transparent" networks have been proposed, which networks employ either high speed optical time division multiplexing (OTDM) or wavelength division multiplexing (WDM) to attain the high data-rate. The transparent optical networks rely on optical switching and routing to maintain a transparent path between the source and destination nodes.
A transparent optical network may lie above the top electronic switched transport layer of a "synchronous digital hierarchy" (SDH). A synchronisation between the traffic on the transparent optical network and the switched transport layer is required for the transparent optical network to be compatible with the SDH. This can be achieved, for example by the use of optical switches at the intermediate network nodes, these switches requiring a clock synchronisation signal from incoming traffic, the synchronisation signal also being used for the demultiplexing of channels in an OTDM system.
The networks may also need to support services with a very broad range of bit-rates from 100 MBit/s (eg. video) to many Gbit/s (eg. multiplexed data). In order to maintain this network transparency, the clock extraction technique used needs to be flexible to bit-rate.
In transmission systems, electronic clock recovery circuits are generally used, conventional techniques using electronic filtering in the post detection circuitry. For instance, a high Q electrical filter may be used to extract the clock component in a received data modulation spectrum. The lack of tunable narrow bandpass electronic filters will, however, introduce an electronic "bottleneck" into the otherwise transparent network. If the modulation spectrum does not contain a clock component, such as in non-return to zero (NRZ) format data, then an additional electronic nonlinearity is needed to generate one. Within transparent optical network architectures, electronic clock recovery techniques are disadvantageous as they are bit-rate sensitive, require the tapping of the optical signal which results in power loss, and can also require wide band electronics.
Methods of all-optical clock extraction, particularly where the clock frequency can be tuned over a wide frequency range would be extremely useful.
The present invention is based on the use of a self-pulsating semiconductor laser. Such a device is known, the self-pulsation being caused by self Q-switching within the device caused by instabilities induced by regions of saturable absorption coupling with regions of high gain. The repetition rate of emitted pulses can be controlled by varying the current to either region of a two-region device, and has been found to vary approximately as 1/I.sup.1/2. In a paper entitled "Conditions for Self-Sustained Pulsation and Bistability in Semiconductor Lasers", J. Applied Physics, Vol 58, number 4, pp 1689-1692 (1985), M Ueno and R Lang have shown, by theoretical analysis, that self-pulsation only occurs at certain ratios of the carrier lifetimes, .tau..sub.g .tau..sub.a, and differential gain, (.delta.g.sub.g .delta.n) (.delta.g.sub.a .delta.n), where the subscripts "g" and "a" refer to the gain and absorbing regions respectively, "g" is the material gain (or loss ) and "n" is the carrier density. In general, pulsations are only likely to occur when the carrier lifetime ratio, .tau..sub.g /.tau..sub.a, is high. It is difficult to achieve this for InGaAsP material due to the high Auger coefficient which reduces the carrier lifetime in the gain region, .tau..sub.g, for high carrier densities.
In a paper entitled "All-optical Timing Extraction using a 1.5 .mu.m Self-Pulsating Multi-electrode DFB LD", Electronics Letters, Vol 24, number 23 pp 1426-1427 (1988), M Jinno and T Matsumoto have demonstrated optical clock extraction at relatively low data rates using self-pulsating laser diodes (SP-LDs). The pulsation frequency of the SP-LDs could be varied by about 10 MHz by changing the current applied to one of the sections.
To demonstrate clock recovery, they used RZ data at approximately 200 MBit/s by injecting about 250 .mu.W of optical data into a SP-LD. They achieved clock recovery for input data patterns with up to 7 consecutive zeros. They noted that, at Multi-GHz operation, self-pulsate on is lost as the carrier density in the gain region of the device becomes so high that the consequent reduction in lifetime .tau..sub.g reduces the carrier lifetime ratio .tau..sub.g /.tau..sub.a. This is a complicated area, in that self-pulsation is affected by the ratio of g.sub.g /g.sub.a as well as .tau..sub.g /.tau..sub.a. Increasing the reverse bias to control the gain ratio also increases the unsaturated absorption and increases g.sub.a. Both these effects tend to move a device out of the self-pulsation regime into the bistable regime, resulting in weaker pulsations.
In embodiments of the present invention, not only can the problem indicated by Jinno be overcome but other, very significant, advantages can be achieved.
The aim of the invention is to provide a method and equipment for optical clock extraction which can operate at relatively high input data rates.
The present invention provides a semiconductor laser diode for use in an optical clock extraction system operating at high signal bit rates, the laser diode comprising a self-pulsating device having a region of saturable absorption coupling with a region of relatively high gain, the carrier lifetime in the saturable absorption region being lower that in the gain region.
Advantageously, the carrier lifetime in the saturable absorption region is selectively reduced with respect to that in the gain region.
Preferably, the carrier lifetime in the saturable absorption region is selectively reduced by selective diffusion of a dopant into the absorption region. In order to achieve said selective reduction of the carrier lifetime, it has been realised that selective doping with a component can be successfully used, for instance Zn doping. The Zn ions act as centres for non-radiative recombination, this reducing the carrier lifetime. Conveniently, the selective diffusion of dopant is carried out such that dopant is present in the active layer of the device but not in the material of the device between the active layer and an n-side electrical contact to the device.
Although this effect has been seen previously, in other contexts, as a disadvantage, an aspect of the present invention is to recognise its usefulness in a SP-LD for high frequency operation. More especially, it has been recognised that selective doping could and should be carried out, in the absorbing region only, and the exploitation of this idea has proved very successful. Indeed, it has been found that a single embodiment of the present invention can provide strong pulsation over a frequency range extending from about 3 GHz to 5.2 GHz, inclusive.
Although distributed feedback (DFB) laser diodes have been used in the past for clock extraction, because of their single mode output, and although Fabry-Perot devices do not generally have a single mode output, which would seem desirable in the present context, in practice it has been found that there is no significant problem in clock extraction in using Fabry-Perot devices. In embodiments of the present invention, the output of the Fabry-Perot device, once locked to an input optical clock component, is in general in a single dominant mode. Fabry-Perot devices also have the advantage that they are relatively simple to fabricate compared with DFB devices. Therefore, preferably, the laser diode comprises a Fabry-Perot structure.
It should be noted that, in the present application, a Fabry-Perot device is taken to be a device in which at least a significant proportion of optical feedback to a laser cavity is provided by a facet in the device structure. This is in contrast to a DFB device in which a significant proportion of optical feedback is provided over the length of the path of optical radiation in the device in use, such as is provided by a grating as commonly used in known DFB devices.
Generally, embodiments of the present invention are suitable for operation on any data having a significant clock component, such as RZ data.
In a preferred embodiment, the laser diode has a self-pulsation frequency of at least 5 GHz.
Advantageously, the laser diode further comprises frequency control means for controlling the self-pulsation frequency of the diode so that the self-pulsation frequency can be controllably varied over a frequency range. The frequency control means may comprise a variable gain current supply to a gain region of the diode.
Advantageously, the overall length of the diode lies in the range of from 250 .mu.m to 500 .mu.m, and the length of the saturable absorption region lies in the range of from 25 .mu.m to 40 .mu.m. In a preferred embodiment, the overall length of the diode is 500 .mu.m, the overall length of the saturable absorption region is 25 .mu.m, and said frequency range comprises at least the range of from 3.2 GHz to 5.2 GHz. Alternatively, the overall length of the diode is 250 .mu.m, the length of the saturable absorption region is 40 .mu.m, and said frequency range comprises at least the range of from 0.8 GHz to 3.5 GHz. The frequency range preferably covers a frequency change of at least 1 GHz and more preferably at least 2 GHz.
Preferably, the laser diode includes an active layer of a III-V compound having a direct bandgap, for example InGaAsP.
The invention also provides an optical clock extraction system comprising a self-pulsating laser diode whose output pulsation .rate will lock onto a clock component of an input data signal over a significant range of frequencies of such a clock component, the range extending to a relatively high frequency, wherein the laser diode is as defined above. Preferably, said range extends to a frequency of the order of 5 GHz or greater. The system may be adapted for operation on digital optical input data having a significant clock component.
The invention further provides an optical signal processing system for processing an optical signal having a clock component, the system comprising means for inputting at least a portion of the optical signal to an optical clock extraction device, and means for inputting a clock output of the optical clock extraction device and at least a portion of the optical signal to an optical signal processing device which processes said optical signal by means of said clock output, wherein the optical clock extraction device is constituted by a laser diode as defined above.
Preferably, the clock output is combined with said at least a portion of the optical signal and the combination is supplied to an optical thresholding device.
In embodiments of the present invention, it is possible to use an opto-electronic device to extract a clock frequency from a digital optical signal, not only at the relatively low repetition rates of the order of 200 or 300 MHz, at which this has previously been done, but even at repetition rates of the order of 5 GHz and above. This means all-optical clock extraction can be carried out at data rates of 5 Gbit/s or even higher. Such clock extraction equipment could find major application in future all-optical high capacity networks.
Embodiments of the invention have shown quite surprisingly good operative characteristics, such as requiring input signal powers for successful clock extraction to be only of the order of 10 .mu.W with stability over consecutive "zeros" in the input signals comparable to that which might be expected in practice, that is, of the order of or 31 consecutive "zeros". This number of "zeros" is quite likely to be present for real data where an incoming signal is for instance an OTDM signal.
Good operative characteristics which have been noted for specific embodiments of the invention can be listed as follows:
i) operation 25 times faster than known devices; PA1 ii) 1/10th input optical signal power necessary compared with known devices; PA1 iii) will operate over a wider wavelength range; PA1 iv) better clock extinction ratio; PA1 v) larger clock power; PA1 vi) largely polarisation insensitive; PA1 vii) operates with long "zero" trains; and PA1 viii) can operate at any clock frequency from 0.8 GHz to 5.2 GHz inclusive.
All-optical clock extraction techniques according to embodiments of the present invention may find application in both all-optical and opto-electronic regenerators, and in switch synchronising in future multi GBit/s optical submarine and terrestrial networks.